1. Field of Invention
This invention pertains telephone line repeaters used in the telephone industry, and particularly to a testable line repeater and a method of testing the same.
2. Prior Art and Other Considerations
Telephone central offices are connected by cables comprising networks of twisted wire pairs. The twisted wire pairs carry parallel transmissions. For example, a twisted wire pair on a first side (side 1) of the network carries transmission from a first central office to a second central office, while a corresponding twisted wire pair on a second side (side 2) of the network carries transmission from the second central office to the first central office.
In accordance with a widely used T-1 transmission format, each side of the twisted wire pair network has transmissions from twenty four channels multiplexed thereon at a rate of 1.544 megabits per second. The transmission signals are encoded in an alternate mark inversion, tri-level return to zero (RZ) format.
Devices known as repeaters are typically installed on twisted wire pairs between the central offices for the purpose of regenerating the signals transmitted therealong. Otherwise, the signals might be so degraded or deteriorated, by reason of the physical distance separating central offices or environmental (e.g., noise) considerations, that the signals are not receivable at the receiving central office. A number of repeaters, perhaps as many as twenty or so, may be installed between two central offices. The distance between each repeater is known as a span.
A repeater typically includes input and output transformers for each side of the twisted pair network. An input signal from the input transformer is connected to an automatic line build out (ALBO) device, which in turn is connected to a regenerator. A regenerated signal from the regenerator is applied to the output transformer for further application to the same side of the twisted pair network.
Equipment faults or other difficulties can arise in transmissions between central offices. For example, the twisted pairs may be cut or severed at any point between the central offices. As another example, one of the repeaters positioned intermediate the central offices may malfunction.
The task of pinpointing a fault or difficulty along a T-1 transmission line can be very formidable. The task typically involves human inspection, on a trial and error basis, of as many repeaters as are necessary to locate the fault or difficulty. In some instances the transmission lines traverse rugged terrain, making access to the repeaters extremely laborious In urban areas, the telephone lines and repeaters are often subterranean. Access to subterranean equipment is generally gained through manholes or the like. In many areas, due to environmental concerns and/or governmental regulations, the cost of obtaining access to the telephone equipment via manhole is substantial. Accordingly, regardless of location, a trial and error approach for locating a fault is perplexing and inefficient.
Various techniques have been proposed for remotely testing T-1 line repeaters. One technique involves connecting all the repeaters between central offices with a separate analog signal line for monitoring analog electrical signals at the repeaters. However, since the repeaters along the network are not individually addressable, should a fault or difficulty arise there is no way to determine at what particular repeater the fault arose.
Another technique for remotely testing repeaters involves assigning each repeater a unique address, for example by means of an electromechanical switch specially installed at the repeater. The central office chooses a repeater to test by transmitting up the network a specially coded pattern of information. Included in the coded pattern of information is the unique address of the repeater to be tested. Upon receipt of the coded pattern of information, the repeater having the addressed included in the coded pattern "loops" to return the specially coded pattern back to the originating central office. If a repeater returns the coded pattern addressed to it, the central office assumes that the repeater, and the network spans between the central office and that repeater, are functioning properly.
Assigning unique pre-set addresses to each repeater presents problems. For example, electromechanical switches can break or malfunction, particularly in the diverse and potentially radically variable climatic conditions in which telephonic equipment is expected to operate. Further, it is likely that, in the presetting of hundreds of switches for repeaters, some addresses will be set incorrectly. Moreover, should it be necessary to reconfigure a network to include additional repeaters at some intermediate point, it may also be necessary to reset the addresses of many of the repeaters (by manual access) in order to reflect the reconfiguration of the network.
Accordingly, in view of the foregoing, it is an object of the present invention to provide method and apparatus for remotely testing T-1 line repeater equipment installed in a telephonic transmission network.
An advantage of the present invention is the provision of method and apparatus for interrogating repeaters without requiring a preset address for each repeater.
Another advantage of the present invention is the provision of method and apparatus for obtaining diagnostic information pertaining to a T-1 line repeater without the use of separate analog wires.